The present invention relates generally to an exhaust gas recirculation system for an internal combustion engine and, more specifically, to an improvement thereof which prevents exhaust gases, introduced into an intake passage downstream of a throttle valve, from flowing upstream toward the throttle valve and impairing its response accuracy by depositing exhaust gas contaminants thereon.
An exhaust gas recirculation system is well known in the art, wherein a portion of exhaust gases is reintroduced into an intake passage of an internal combustion engine from an exhaust passage thereof to recirculate the exhaust gases through the engine. FIG. 1 shows a conventional structure used in an exhaust gas recirculation system. An intake passage 1 extends from an air cleaner inlet to four engine cylinders, passing through a carburetor and an intake manifold 2, to introduce an air-fuel mixture to each engine cylinder. The intake manifold 2 has four branches 3 and a common riser 4 upstream of the juncture of the branches 3. The carburetor including a throttle valve for controlling the intake flow of the air-fuel mixture toward the engine cylinders is mounted on the upstream end surface 5 of the common riser 4 by way of a heat insulator. An exhaust gas recirculation (EGR) pipe 6 is perpendicularly inserted into the common riser 4 through an opening 7 formed in the wall of the riser 4 and its free ends opens into the intake passage 1. The other end of the EGR pipe 6 receives exhaust gases fed from an exhaust passage for recirculation of a portion of the exhaust gases into the intake passage.
FIG. 2 shows another conventional structure used in the exhaust gas recirculation system, wherein like or corresponding parts or members are designated by the same reference numerals as in FIG. 1. An EGR pipe 8 is perpendicularly inserted into the riser 4 through the opening 7 formed in the wall of the riser 4. The EGR pipe 8 crosses the intake passage 1 and fixedly engages at its one end with a recess 9 formed on the inner surface of the riser 4. The other end of the EGR pipe 8 receives the exhaust gases fed from the exhaust passage as in the foregoing conventional structure. The wall of the riser 4 is formed with four passages 10, each connecting the opening 7 or the recess 9 to one of four recesses 11 formed in the upstream end of the riser 4, as seen in FIG. 3. The EGR pipe 8 is formed with four apertures 12, each corresponding to one of the four passages 10 so that the exhaust gases fed from the exhaust passage is introduced into the intake passage 1 via the apertures 12, the passages 10 and the recesses 11. As in the structure of FIG. 1, the carburetor with the throttle valve is mounted to the upstream end surface 5 of the riser 4 by way of a heat insulator.
In the conventional structures of FIGS. 1 and 2, however, the exhaust gases are introduced into the intake passage 1 in a direction substantially perpendicular to the intake flow of the air-fuel mixture, or in a direction substantially against the intake flow. As a result, the exhaust gases flow upstream toward the throttle valve and impair its response characteristics by depositing exhaust gas contaminants thereon. In addition, in the structure of FIG. 2, forming the recesses 11 and the passages 10 complicates manufacture, and the heat insulator to be placed between the upstream end surface 5 of the riser 4 and the carburetor must be more heat-resistant than is required in the structure of FIG. 1, with the result that manufacturing costs increase.